Nedarim, Chapter 3, Mishnah 6. This mishnah, and those through the end of the chapter, deal with the various expressions a person may use when making a vow: who is included in those vows and who is not.
One who vows against seafarers:
A person who vows not to derive benefit from "yordei hayam" - seafarers, meaning those who travel by ship - his vow applies only to those who travel by ship, and it does not apply to "yoshvei hayabashah" - the dwellers on dry land, those people who remain on land and do not go down to the sea. This is clear, since there are people who never board ships at all.
One who vows against those who dwell on dry land:
By contrast, one who vows against "yoshvei hayabashah" - the dwellers on dry land - his vow includes seafarers as well, since even those who go down to the sea in ships are included among those who live on dry land. Living on dry land is the common denominator of all people, whereas going down to the sea in a ship is not something shared by everyone.
The mishnah concludes by establishing that the discussion here is not about those who sail from Akko to Yafo - a short voyage considered local - but rather about those whose practice is to set out on lengthy sea voyages. That is what the mishnah intends by this wording.
The dispute in the Gemara: to what does the qualification at the end of the mishnah refer?
One opinion - the qualification refers to the first clause: In the first clause it was stated that one who vows against seafarers is permitted to derive benefit from those who dwell on dry land, and by implication we learned that he is forbidden to derive benefit from seafarers. The concluding words then come to define who those seafarers of the first clause are: the reference is not to those who travel from Akko to Yafo, that short voyage, and they are not forbidden by this vow. It emerges that the final clause comes to be lenient and to exclude, teaching that we do not include them in the vow.
A second opinion - the qualification refers to the final clause: In the final clause it was stated that seafarers are included among those who dwell on dry land, and the concluding words come to teach that the reference is not only to those who make the short voyage from Akko to Yafo, who are considered dwellers on dry land, but even those who set out on the longest of voyages are likewise included among the dwellers on dry land and are forbidden by his vow. According to this interpretation, the concluding words come precisely to be stringent and to widen the circle of those included in the vow.
In summary: In this mishnah we learned that one who vows against seafarers is permitted to derive benefit from those who dwell on dry land, while one who vows against those who dwell on dry land is forbidden even with regard to seafarers, because living on dry land is a common denominator for everyone. We further learned that the mishnah is not dealing with the short voyage from Akko to Yafo but with lengthy sea voyages, and the Gemara records a dispute as to whether this qualification refers to the first clause and comes to be lenient, or to the final clause and comes to be stringent and to broaden the circle of those included in the vow.